[joshuamorton]

When discussing conditional probability, this is absolutely what you can do.

Let's use your example. Conditional probability is P(A|B), the probability of event A, given that event B was observed. What's the probability that I am a magician, given that I shuffled the deck and when you saw it it was still in new deck order?

Now certainly, there is an astronomically small chance that this was observed due to random chance. And yet if you observed this, I'd expect that you would, with relatively high confidence, believe that I stacked the deck.